This invention relates to a method and apparatus for assisting by fluid flow the doffing of an overlying porous layer of particulate matter (referred to as a "mat") from the surface of a filter medium, such as a fluid permeable filter screen. The method and apparatus according to this invention has application in many different types of condensers and fluid filters. The particular disclosure set out in this application relates, for illustrative purposes, to the fluid assisted doffing of a mat comprised of various types of particulate matter, such as fibers, dust and dirt commonly found in the textile processing industry and which for reasons of safety and health must be removed from the environment. This is usually accomplished by filtering the air within which the dirt and fibers are entrained through a filtering apparatus.
One of the ways this is accomplished is by filtering the particulate laden air through a filter medium such as a screen. Filtration takes place not only by the passage of the air through the filter medium, but after a mat of particulate matter builds up on the filter medium, the mat itself acts as an additional filter medium of enhanced filtering capacity. Prior inventions by the present assignee have dramatically increased the efficiency of such filters by optimizing the size of the openings in the filter medium and then using the filter medium essentially as a filter support on which an overlying porous mat having enhanced filtering capacity is built.
However, the mat must be removed periodically from the underlying filter support so that the thickness of the mat does not substantially exceed a point where the filtration efficiency ceases to improve. This is also necessary because the increasing thickness of the mat progressively restricts the flow of air through the filter, causing filtration capacity at any given energy level to decrease. When the mat is removed from only a portion of the filter screen, the pressure drop through this area decreases relative to the portion of the filter screen still carrying the mat. As a result, most of the air entering the filter will move toward the area of least resistance and pass through the clean filter medium. This results in a greatly reduced filtration efficiency through this area until the mat again begins to accumulate on the underlying filter medium.
This known deficiency in prior art filtering processes has been adapted and turned into a useful phenomonen in the recent U.S. Pat. No. 4,226,715 issued to the present assignee, Terrell Machine Company. As disclosed in the above-referenced patent, particulate laden air is used to build an overlying mat of enhanced filtering capacity over a filter screen which acts primarily as a filter support for the mat. Because of the structure of the filter, air is not permitted to exit the filter enclosure until it has passed through this overlying mat. Therefore, periodic decrease in filtered air quality as a result of some air passing through the clean filter screen has been eliminated.
However, in order for a mat to be quickly and efficiently built up over the doffed portion of the filter screen, the screen must be substantially cleaned each time it is doffed. If this is not done, areas of the filter screen may remain blocked by contaminants which will prevent air from passing through that section of the screen and will therefore prevent the proper buildup of the mat. This occurrence increases the amount of energy required and decreases the filtration capacity of the filter. Should the blockage become dislodged at some later stage in the filtration process, a substantial decrease in filtering efficiency would occur as the air is passed through this bare area instead of through the overlying mat, as is desired.
Certain materials are inherently easy to doff under optimum conditions from an underlying filter screen. For example, textile fibers accompanied by relatively low percentages of dust and dirt form an overlying mat which has a high degree of internal cohension. Thus a conventional doffing roll having radially extending elastomeric vanes can easily remove the entire mat leaving a completely clean filter screen. However, in conditions of high humidity and/or with a mat having a realtively large percentage of dust and dirt, the action of doffing the mat can produce a very thin layer of mud-like substance which coats the filter screen and completely blocks large areas of the screen against the passage of air. The presence of fibers in this "mud" provides further strength, causing it to cling tenaciously to the filter screen. This problem is even more serious in environments where the particulate matter being filtered from the air is comprised principally of dust, dirt, fly ash, flour or the like.
Experimentation with this problem has resulted in the development of a way to prevent blockage of the filter screen. The mat is pushed away from the screen by passing a stream of air through the screen and against the underside of the mat. This has been accomplished while completely preventing the passage of any contaminated air into the environment.